Production of lower aliphatic acids



July 23, 1957 A. ELCE ET AL PRODUCTION OF LOWER ALIPHATIC ACIDS FiledOct. 9, 1953 2,800,504 PRODUCTION OF LOWER ALIPHATIC Actus Alec Elce,Banstead, Ian Kenneth Miles Robson, Clapham Common, London, and DonaldPeter Young, Sanderstead, England, assignors to The Distillers CompanyLimited, Edinburgh, Scotland, a British company Application October 9,1953, Serial No. 385,182

Claims priority, application Great Britain October 15, 1952 7 Claims.(Cl. 26d- 533) The present invention relates tothe production ofaliphatic acids, particularly acetic acid.

It has been found that when oxidising parafiinic hydrocarbons containing4-8 carbon atoms at temperatures above about 130 C. and subsequentlydistilling the product that there are obtained as successive'fractions:(l) volatile non-acidic Vorganic compounds boiling in the presence ofwater in the range about C. or lower up to about 99 C. (called hereinlight ends`); (2) aliphatic acids principally of one to four carbonatoms; and (3) high boiling residues. It has further been found that thelight ends fraction contains a substantial proportion of water togetherwith traces of aldehydes andvarying proportions of ketones, alcohols,esters and other compounds including unreacted hydrocarbons. It is knownthat certain of these compounds, such as aldehydes and some ketones, maybe oxidised to yield acids, while others such as alcohols and esters arenot easily Oxidised, particularly when water is present.

It is an object of our invention to oxidise in liquid phase paraimichydrocarbons of about four to eight carbon atoms, with gases containingmolecular oxygen, to produce aliphatic acids of one to four carbonatoms. It is a further object of our invention to oxidise volatilenonacidic oxidation products in such a way that further yields of acidsof one to four carbon atoms may be obtained therefrom Vwithout thenecessity for effecting prior separation of said volatile non-acidicoxidation products into their constituents.

lt has now been found that by oxidation of these light ends in theliquid phase, under conditions similar vto those used for the oxidationof the hydrocarbon,A there lare obtained further useful amounts of thedesired acidproducts ,of which acetic acid is the main constituent.

It has now further been found that although the light cnrs when oxidisedseparately yield a substantial amount periods without build up ofunoxidisable material in the reaction system and without any evidence ofa decline in the rate of production of acids expressed on a reactorvolume basis. This is surprising as it was expected that in any processfor the oxidation of parains to yield acids it would be necessary toremove the unoxidisable constituents of the light ends by one means oranother from the process, in order to prevent accumulation ofunoxidisable constituents in thesystem and a consequent lowering of theoutput of the desired acidic products.

The invention accordingly comprises the novel processes and steps ofprocesses, specific rembodiments of which-are described hereinafter byway of example and in accordance with which `we now preferto practicethe invention. y`Accordingly, the present invention is for a process forthe production of acetic acid and other aliphatic acids which comprisesoxidising in the liquid phase a -parain hydrocarbon lof four to eightcarbon atoms' with a gas the liquid phase. The present invention alsocomprises the above process including the step of adding fresh paraffinhydrocarbon to the light ends and thereafter oxidising said mixture.

The oxidation of the parafn hydrocarbons of four to eight carbon atomsis suitably carried out for example as described in copendingapplication No. 385,272, Habeshaw et al., filed October 9, 1953.According to application No. 385,272 a paraihn hydrocarbon fractioncontaining hydrocarbons of 4 to 8 carbon atoms, at least 40% of saidfraction consisting of hydrocarbons of 6-8 carbon atoms, wherein atleast 40% of the'parans of 6-8 ca rbon atoms of said fraction consist ofbranched chain parafns having methyl substituents, and wherein saidfraction is of boiling range not exceeding 95 C., is oxidised in theliquid phase with a gas consisting of or comprising molecular oxygen.

The n parain hydrocarbon of four to eight carbon atoms employed for theoxidation is desirably one which does not exert too high a vapourpressure, specically those which are normally liquid at roomtemperatures of about 30 C. and atmospheric pressure. p

y lt has been found that the reaction mixture is normally a homogeneousliquid at operating temperature,'but that on cooling two liquid layersseparate. Of these the upper layer is mainly unchanged hydrocarbon,while the lower layer contains the bulk of the acidic products andwater. It is, therefore, a feature of the process of the presentinvention to operate by removing continuously or intermittently a partof the reaction mixture as a homogeneous liquid, cooling this materialto a temperaturebelow about C., and preferably to as near roomtemperature as may be convenient, separating the liquid layers,returning all the upperlayer to the reactor and removing at least partof this lower layer as the product.

The primary oxidation product may be distilled to separate as distillatethe light ends, for example the fraction which under batch distillationconditions will boil over the range of about 30 C. to about 99 C. Duringthe latter part of the distillation under these conditions some of thewater separates asa lower layer in the distillate, and this may'beseparated and removed if so desired. Separation of the light ends mayalso,however, be carried out in a continuous manner, employing acontinuous still operated with a head temperature of about 65-70 C., anda base temperature of about 105 C. In this case no separation of anaqueous layer from the distillate normally occurs.

The products of oxidation may be worked up to vyield further light ends,which may conveniently be termed secondary light ends, acids of -one tofour carbon atoms and higher boiling material in the same way as theproducts from the primary oxidation of the hydrocarbon. The secondarylight vends so recovered may again be oxidised, although it will befound that these yield a lower proportion of acids than the primarylight ends, and that a higher proportion of the charge is recovered asunoxidised light ends. While therefore the process may be repeated asoften as desired, it will probably be found uneconomic to repeat theprocess more than two or three times. Alternatively the secondary lightends recovered from the secondary oxidation may be combined with a freshbatch of primary light ends and the mixture oxidisred or they may becombined with the feed of primary light ends in a continuous oxidationprocess for oxidation of light ends.

According to a preferred embodiment of the invention light ends aremixed with fresh hydrocarbon, suitably with an amount of the latterequal to the balance of the first product after removal of the lightends therefrom and the mixture again oxidised. VVery suitably on mixingthe light ends with fresh hydrocarbon, the aqueous phase which forms isseparated and discarded, and the mixture of light ends and freshhydrocarbon of reduced water content oxidised. This second oxidationproduct may again be distilled to recover light ends, and the processcan be repeated indefinitely. It will be realised that the only liquidproducts resulting from the overall process are those remaining asstill-base product after distillation to separate the light ends, andthese mainly comprise the desired acids of one to four carbon atoms. Itis preferred, however, to operate continuously by introducing theparafnic hydrocarbon feed together with air or oxygen into an oxidationreactor, withdrawing oxidation products from the reactor, subjecting theproduct to distillation whereby light ends are separated as an overheadfraction and returning these to the oxidation reactor in conjunctionwith the feed of fresh hydrocarbon.

The oxidation process of the present invention, mainly the oxidation ofthe parain hydrocarbon, of the the light ends, or of the mixture oflight ends and parain hydrocarbon may each be carried out under similarconditions.

rllhese oxidations may be effected with any gas containing molecularoxygen, whether in the form of air or of mixtures poorer or richer inoxygen than is air; part of the molecular oxygen may, if desired, be inthe form of ozone. The use of superatmospheric pressures in theoxidation will generally be necessary in order to maintain a major partof the reactants in the liquid phase.

The temperature of oxidation should be suiciently Vhigh to provide aneconomically attractive rate of oxida- `tion and a low level ofperoxidic compounds.

Thus, the temperature should not be so low that insutiicient oxidationto acids occurs or alternatively be so high that excessive oxidationoccurs of the hydrocarbons, or of their primary oxidation products, tooxides of carbon and water. Temperatures in the range about 130 C. to200 C. have been found suitable, although higher or lower temperaturesmay be used if so desired.

These oxidations may be carried out in a variety of types of apparatus,provided only that the reaction mixture is maintained substantially inthe liquid phase, and that adequate contact is maintained between theliquid reaction mixture and the oxidising gas. Thus, the oxidation maybe carried out in a pressure reactor fitted with a mechanical stirrerfor agitation and dispersion of the gas throughout the liquid.Alternatively, the reactor may be in the form of a vertical tower withthe feed of oxidising gas at the base, or at a number of points up thetower, in which case the necessary agitation is effected by the gasitself; in this case it is advantageous to subdivide the gas feed bymechanical means to obtain a line dispersion at the point or points ofentry and throughout the reaction zone. Alternatively, the oxidising gasmay be fed into a stream of liquid moving with high velocity in acirculatory system, such as a coil reactor with forced circulation.

These oxidations may be carried out in the absence o f any addedcatalyst, or alternatively if so desired, the oxidation may be carriedout in the presence of a suitable oxidation catalyst. Such catalystsinclude compounds of those metals that are capable of existing in morethan one valency` state. Examples of suitable metals include manganese,cobalt, nickel, vanadium and copper. These catalysts may be convenientlyadded 'n the form of their oil soluble salts with organic acids, oralternatively the catalyst metal may be added in the form of an anion,whether as the free acid or a salt thereof, for example as a vanadate.

It is preferred to employ materials for the construction of the reactorand oxidation equipment which, pos- Example 1 The oxidation apparatusconsisted of a stainless steel reactor fitted with a stirrer, a bottomair inlet, a line for removing waste gas, connections for the additionand withdrawal of liquid and means for the measuring and controlling ofthe reaction temperature. Provision was made to withdraw continuously apart of the liquid products through a cooler to a separator, from whichthe lower layer could be withdrawn as product and the upper liquid layerreturned to the reactor.

The reactor was charged with 1.5 litres of n-heptane containing 0.01%W./v. of manganese dissolved in the form ofthe naphthenate. Thetemperature was raised to 160 C. and the air feed was started andmaintained at a rate of about 7 litres/minute throughout the experiment.

Circulation of the reactor contents through the cooler `and separatorwas controlledA at about 0.3 litre/hour. The lower liquid layerappearing in the separator was removed as product and replaced withfresh n-heptane, sufficient to keep constant the total volume in thesystem, at an average rate of 0.18 litre/hour.

The Voxidation was continued for a total of 50 hours under the aboveconditions. The product was distilled batchwise to recover as firstfraction light ends boiling from 21 to 95 C., followed by a mixture ofacids of one to four carbon atoms and water. 'I'he latter fraction wasdehydrated and distilled to yield the individual acids in successivefractions.

Overall yields from the oxidation, expressed as grams from grams ofn-heptane converted, were as follows:

Grams/100 grams n-heptane consumed Light ends 25.9

Formic acid v 3.3 Acetic acid 27.5 Propionic acid 9.9 Butyric acid 6.8

Total C1C4 aliphatic acids 47.5

Distillation residues 31.2

1283 grams of light ends thus obtained were oxidised in the apparatusused above, except that the reaction mixture was not circulated throughthe cooler and separator. These light ends contained by analysis 0.34equiv/100 g. of saponiable compounds and 0.51 equiv./ 100 g. of carbonylcompounds. The charge contained 0.01% w./v. manganese dissolved as thenaphthenate.

The reactor pressure was increased to 200 lbs/sq. in. and the air feedstarted. On raising the temperature to 60 C. absorption of oxygenstarted but it was soon necessary to increase the temperature to about168 C. to maintain the rate of oxidation. After 7.5 hours total durationthe presence of substantial amounts of oxygen in the waste gas indicatedthat oxidation was complete, and the operation was concluded.

The liquid reaction products were distilled as in the rst step, toseparate first a secondary light ends fraction, followed by a mixture ofaliphatic acids of one to four carbon atoms and water. Furtherdistillation of the wet ac ids fraction yielded the following overallresults, ex-

the oxidation.

Y pressed .grams per 10o .artigli-tends Acharged' fo Example A straightrun gasoline fraction v`b.`:p. --.95 C. from Middle East petroleum'iwasoxidisedhnde'r continuous conditions according totheme'thod described inVExample 1. The hydrocarbon chargd'and 'fed contained 0.01% manganesedissolved a's the naphtheate. .'Ihe `temperature and pressure of theoxidationwerel'SOlO" Cfand 300 lbs/sq'. in. respectively,'the rate ofvcirculation'ofthe reactor contents through the `eoolerand separatorbeing about 2.1 litres/hour, and the 'rat'e'of fair 'feed totheoxidation about 1 1'2 litres/minutes. lThe :feed rate `of freshhydrocarbon to the system 'was about "0.43V litre/ hour, being the.amount necessary to fbalancethe'frate of withdrawal of the loweraqueous-'acid layer 'o'fthe cooled product. R .1.1.

The oxidation was continued fora total of 102 hours, and the product wassubjected to" distillation in a continuousstill, consisting of a `72`x"1.5 inch Yeoluniii packed with glass helices, the point "of feedbeing halfway 'up "the column. The 'still' was `operated at atmosphericpressure with temperatures Vnr85-87" C. a'tfthe heaeffnu 10e-10s C. atthe base. The liquid feed 'rate 110 `litres/hour and the refluxratioab'outZal, the liglitfehds' btain'edas overhead distillate being45.5% byw'eig'htf the vfeed. The base product consisted of'a mixture ofl'vjt'at'er,fali phatic Vacids of one to `four carbon atoms andresidues, and was separated into its various Acons tituents `by furtherdistillation.

Overall yields from the oxidation, vexpressed -as grams of recoveredmaterials from 100 grams of hydrocarbon consumed, were as follows:

f .Grams/IOO-granis hydrocarbon consumed The light ends thus obtained bydistillation were oxidised batchwise according to the method of Example1 for the oxidation of light ends, at a temperature of 150- 170 C. and apressure of 300 lbs/sq. in. The duration of a number of batchexperiments, up to the point where absorption of oxygen from the airfeed had virtually ceased, was 7-13 hours according to the rate at whichthe temperature was increased.

The oxidation products were combined and distilled in a continuous stillto remove light ends and the still base product subsequently distilledto separate the acids as described above. Overall yields of materialsrecovered expressed as grams per 100 grams of light ends charged to theoxidation were as follows:

Grams/ grams light ends c arged ASecondary light ends 21.7

.Formic acid f 8.4 Acetic acid 40.4 l VPropion'ic acid 6.5 Blityric acid1.6

.Total C1C4 aliphatic acids 56.9

Distillation residues 4.6

' Example 3 The secondary light ends recovered in Example 2, whichcontained by analysis 0.30 equivalent/ 100 grams of 'saponiliablecompounds and 0.91 equivalent/ 100 gram-s of carbonyl compounds, werefurther oxidised 'batchwise according to the method of Example 1 for theoxidation of light ends, at a temperature of 174 C. and 300 lbs/sq. in.pressure. The oxidation proceeded for v3.5 lhours 'after which theabsorption of oxygen had vir- 'tually'ceased 'Ihe Aoxidation product wasdistilled, and the 'yields were as follows:

Grams/ 100 rams light The oxidation reactor is shown diagrammatically inlthe accompanying ligure and consists of a Vertical stainless steel tube1 '2.6 inches in diameter and 6 feet high,

having air feed points 2 at the base and about the midpoint. Fresh`hydrocarbon is introduced continuously by lin'e 3 through aipreheater13. Both gaseous and liquid reaction products are removed from thereactor by line w4, and'after coolingV in cooler 5 are fed to gas/liquidseparator 6, from which the waste gases are withdrawn Vby line 7. Theliquid products pass to liquid/liquid separator 8, where the loweraqueous acid product phaseis allowed to settleas a lower layer, part ofthis layer being withdraw'as product'throu'gh line 9. The upperhydrocarbon layer is returned to the reactor through preheater 10, withpart of the lower separated layer. The whole aqueous acid product is fedby line 9 through a preheater 10A to the mid-point of a continuousdistillation column 11 which is packed with JA inch rings and has theequivalent of 5 theoretical plates above and below the feed point,provided with a reboiler 11A and cooler 11B as shown. The column isoperated with a reflux ratio of 4: 1, the temperatures beingapproximately 66 C. at the head, approximately 82 C. at the feed point,and 104- C. in the reboiler. The total distillate is returned by line 12to the oxidation reactor after admixture with the fresh hydrocarbonfeed. The base product from the still is withdrawn by line 14, andsubmitted to further distillation to recover the acids formed as mainproduct.

During the steady oxidation period of hours, at a temperature of -175C., and a pressure of 600 lbs./ sq. in. with a waste gas rate of 1800litres/hour (as free gas containing 0-1% oxygen), a hydrocarbon feedconsisting of a straight run gasoline fraction, B. P. 20-95 C., fromMiddle East petroleum, was fed into the reactor at the rate of 510grams/hour. Light ends, recovered as distillate from the continuousstill at the rate of 432 grams/hour, were fed back completely to thereactor and thus constituted 46% of the total feed to the oxidationreactor. No decrease in the rate of oxidation was noted during theoxidation, the production of acids of one to four carbon atoms remainingconstant at S6 grams/hour! litre of reactor space. The base product fromthe continuous still was drawn off at a rate of 565 grams/hour, andcontainedabout 60% of acids of one`to four carbon atoms, practically allthe remainder consisting of water and substances boiling above butyricacid. At the end of this experiment the aqueous product in theV reactorcontained 36% w./w. of light ends, showing that no accumulation of lightends had occurred. The yield of acids of one to four carbon atoms,isolated by fractional distillation of the base product from thecontinuous still, was 66 parts per 100 parts of hydrocarbon consumed(including hydrocarbon removed in the waste gas stream). Of the acids ofone to four carbon atoms recovered, about 75% by weight consisted ofacetic acid.

By way of comparison, in oxidations carried out under similar conditionsin the same oxidation vessel using the same hydrocarbon feed, butwithout recycle of the light ends to the oxidation, the correspondingyield of acids of one to four carbon atoms (of which about 70% by weightwas acetic acid) was about 40 parts per 100 parts of hydrocarbonconsumed.

We claim:

l. A process for the production of lower aliphatic acids vwhichcomprises oxidising in the liquid phase a parafiin hydrocarbon of fourto eight carbon atomswith molecular oxygento produce lower aliphaticacids and light ends, distilling the oxidation product to separate aslight endsthe materials ,boiling below 99 C. in thev presence of water,admixing said light ends with a parafin hydrocarbon of four to eightcarbon latoms, thereafter' oxidising in the liquid phase this mixturewith molecular oxygen without substantial accumulation of light ends toproduce lower aliphatic Vacids at a greater ratio of said acids tohydrocarbon consumed than in such oxidation of the hydrocarbon alone.

2. A process for the production of lower aliphatic acids as claimed inclaim 1, wherein the oxidation product of the said mixture is distilledto separate the materials boiling below 99 C. in the presence of water,and thereafter admixing said materials with a paraffin hydrocarbon offour to eight carbon atoms and oxidising in the liquid phase withmolecular oxygen to produce lower aliphatic acids.

3. A process for the production of lower aliphatic acids which comprisesoxidising in the liquid phase a parain hydrocarbon of 4 to 8 carbonatoms with molecular oxygen to produce an oxidation product containinglower aliphatic acidsgand light ends, cooling the oxidation product to atemperature below C., to allow it to separateinto .two layers, returningthe upper layer contain- 4ingf'unreaet'ed hydrocarbon to the oxidationzone, distilling the lower layer vto separate as light ends thematerials boiling below 99 C. in the presence of water, mixing said,light ends with a parain hydrocarbon of 4 to 8 carbon atornsandthereafter oxidising in the Vacid which comprises oxidising in theliquid phase a paraffin hydrocarbon of four to eight carbon atoms withmolecular oxygen to produce lower aliphatic acids, separating at leastpart of the oxidation product, distilling said separated oxidationproduct to separate as light ends the material boiling below 99 C., inthe presence of Water, returning said light ends to the parainhydrocarbon supplied to the oxidation zone, thereafter oxidising intheliquid phase this mixture with molecular oxygen without substantialaccumulation of light ends to produce acetic acid at a greater ratio ofacid to hydrocarbon consumed than in such oxidation of the hydrocarbonalone, and continuously recovering the acetic acid from-the separatedpart of the oxidation product boiling above 99 C. in the presence ofwater.

5. A continuous process for the production of acetic acid as claimedinvclaim 4, wherein the separated oxidation product is cooled to atemperature below 80 C. and allowed to separate into two layers, theupper layer returned to the oxidation zone and at least part of thelower layer subjected to the said distillation process.

6. A continuouSprocess for the production of acetic acid as claimed inclaim 4 wherein the light ends being returned to the oxidation zone aremixed with the hydrocarbon being employed, the aqueous phase formed,separated and discarded and the mixture of light ends and hydrocarbonpassed to the oxidation zone.

7. A processV for fthe production of acetic acid as claimed in claim 4wherein the oxidation is carried out at a temperature in the range13D-200 C.

References Cited in the le of this patent UNITED STATES PATENTS

1. A PROCESS FOR THE PRODUCTION OF LOWER ALIPHATIC ACIDS WHICH COMPRISES OXIDISING IN THE LIQUID PHASE A PARAFFIN HYDROCARBON OF FOUR TO EIGHT CARBON ATOMS WITH MOLECULAR OXYGEN TO PRODUCE LOWER ALIPHATIC ACIDS AND LIGHT ENDS, DISTILLING THE OXIDATION PRODUCT TO SEPARATE AS LIGHT ENDS THE MATERIALS BOILING BELOW 99*C. IN THE PRESENCE OF WATER, ADMIXING SAID LIGHT ENDS WITH A PARAFFIN HYDROCARBON OF FOUR TO EIGHT CARBON ATOMS, THEREAFTER OXIDISING IN THE LIQUID PHASE THIS MIXTURE WITH MOLECULAR OXYGEN WITHOUT SUBSTANTIAL ACCUMULATION OF LIGHT ENDS TO PRODUCE LOWER ALIPHATIC ACIDS AT A GREATER RATIO OF SAID ACIDS TO HYDROCARBON CONSUMED THAN IN SUCH OXIDATION OF THE HYDROCARBON ALONE. 